Manufacturing methods of flexible bonded magnet and motor using the same

ABSTRACT

Methods for manufacturing a flexible bonded magnet, and a high-efficiency small motor using the magnet are disclosed. The flexible bonded magnet is manufactured through the processes of compressing a new compound consisting of flexible thermosetting resin composite and magnetic powder, which contains thermosetting resin, thermoplastic resin, etc.; heat-curing a green sheet derived from the above process; and rolling.

TECHNICAL FIELD

The present invention relates to a flexible bonded magnet produced fromrare earth system magnetic powder, and a method of manufacturing a motorusing the flexible bonded magnet.

BACKGROUND ART

Quite a number of small motors are used in personal computers and theperipheral devices, IT-related appliances and the like apparatus in thefield of advanced technologies. These motors are requested to be morecompact in the size, higher in the output and efficiency, along with theongoing pursuit of compactness and lightness in the appliance sector.Among the total production number of small motors, DC motors account forapproximately 70%. Most of the general-use DC motors use rubber bondedferrite magnets. High-performance DC motors employ ring-shaped bondedmagnets produced by compressing a composite of magnetically isotropicNd—Fe—B system magnetic powder and a rigid thermosetting resin such asepoxy resin. Technological developments in the small motors representthe improvements achieved in the field of bonded magnets manufactured bybonding a magnetic powder with a binder.

FIG. 13 describes the technology combinations used for manufacturingconventional bonded magnets. The magnetic powder can be a hard ferritesystem magnetic powder, an Alnico system magnetic powder or a rare earthsystem magnetic powder. The binder can be a flexible resin (e.g. rubber,thermoplastic elastomer), a rigid thermoplastic resin or a rigidthermosetting resin. The forming process can be a calendering, anextrusion, an injection molding or a compression. Conventionalcombination of these items is indicated with solid lines.

It has been known that the rare earth magnetic powders can be combinedwith a flexible resin, a rigid thermoplastic resin and a rigidthermosetting resin. It is also known that the rare earth magneticpowders can be combined with a calendering, an extrusion, an injectionmolding or a compression. Namely, the rare earth magnetic powders areknown to be compatible with any one of the items described in thebinders and the processes. In the combination of compression process andrare earth system magnetic powder, however, the binder is limited to arigid thermosetting resin, such as an epoxy resin.

Meanwhile, there are following disclosures in the flexible bonded magnetsector for use in small motors whose output power is lower than severaltens of watts, which being subject of the present invention: JapanesePatent No. 2766746 and Japanese Patent No. 2528574 disclose bondedmagnets produced by a process of rolling magnetic materials made of rareearth element system magnetic powders and flexible resins. Permanentmagnet type motors using the above sheet-formed flexible bonded magnetsare also disclosed. However, the maximum energy product (hereinafterreferred to as MEP) of these bonded magnets is in the level as low as 50kJ/m3. Japanese Patent Laid-Open Application No. H5-299221 teaches amethod of manufacturing a bonded magnet by rolling a mixture of rareearth-iron-nitrogen system magnetic powder and flexible resin. MEP ofthis bonded magnet is 42 kJ/m3. Japanese Patent Examined Publication No.H6-87634 discloses a bonded magnet manufactured by compressing amagnetically isotropic R—Fe—B (R signifying at least either one of Ndand Pr) rare earth system magnetic powder and a rigid epoxy resin. Apermanent magnet type motor using a multipole-magnetized ring magnet isalso disclosed. MEP of which is 77 kJ/m3.

In order to implement a bonded magnet for small motors that is moreefficient than the above-described conventional ones, it is essential tohave a new binder system and an optimized processing. Specificcharacteristics needed for the new binder system are that it has astrong adhesive strength and the density of bonded magnet can be raisedeasily. Insufficient adhesive strength causes partial fracture of magnetbody, and the fractured particle would scatter inviting serious damages.If the density is not high enough, the magnetic characteristics do notimprove. The present invention aims to solve the above problems, andoffer a method for manufacturing a highly-efficient and reliable bondedmagnet for use in small motors. A method for manufacturing permanentmagnet type motors using the bonded magnet is also disclosed in thepresent invention.

DISCLOSURE OF INVENTION

A method for manufacturing a flexible bonded magnet is offered. Itincludes process steps of compressing a compound made of rare earthsystem magnetic powder and flexible thermosetting resin composition, i.ea composite curing with heat a green sheet provided through the aboveprocess, and rolling the green sheet. The thermosetting resin compositeincluding a solid epoxy oligomer in the normal room temperature and apolymide powder having thermo-compression bonding property provided withstickiness in the normal room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a manufacturing process chart of a flexible bonded magnetin the present invention.

FIG. 2 is a chart showing a relationship between the density of greensheet and the forming pressure in the present invention.

FIG. 3 is a chart showing a relationship between the density of flexiblebonded magnet and the contained amount of magnetic powder in the presentinvention.

FIG. 4A is a chart showing a relationship between the green sheet curingtemperature and the tensile strength of flexible bonded magnet in thepresent invention.

FIG. 4B is a chart showing a relationship between the green sheet curingtime and the tensile strength of flexible bonded magnet in the presentinvention.

FIG. 5 shows a relationship among the rolling rate in the rolling, thewindability of a rolled flexible bonded magnet around a mandrel and thedimensional change caused as the result of rolling, in the presentinvention.

FIG. 6 shows a relationship among the rolling rate, the elongation of aflexible bonded magnet and the tensile strength, in the presentinvention.

FIG. 7 is a chart showing a relationship between the amount of magneticpowder contained in flexible bonded magnet and the elongationbefore/after the rolling, in the present invention.

FIG. 8 compares the demagnetization curves of a flexible bonded magnetin the present invention, a conventional ferrite bonded magnet and aconventional rare earth bonded magnet.

FIG. 9 compares the B-H curves of a flexible bonded magnet in thepresent invention, a conventional ferrite bonded magnet and aconventional rare earth bonded magnet.

FIG. 10 shows the demagnetization curves of other flexible bondedmagnets in the present invention produced from different rare earthmagnetic powders.

FIG. 11 is a chart showing the temperature dependence of initialirreversible flux loss in the present invention.

FIG. 12 shows a cross sectional view of a DC motor incorporating aflexible bonded magnet in the present invention.

FIG. 13 shows conventional elementary technology items used formanufacturing bonded magnets.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is preferred in the present invention to constitute a binder systemwith an solid epoxy oligomer in the normal room temperature, a polyamidepowder having stickiness in the normal room temperature and a latentepoxy hardener in powder state. The stickiness of polyamide powder inthe present invention is provided by applying an adhesive agent, atackifier, a plasticizer, etc. It works to fix rare earth element systemmagnetic powder and binder together in the compound state beforecompression.

The polyamide powder means in the present invention at least either oneof polyamide powder and polyamide-imide powder. Nylon 6, nylon 66, nylon610, nylon 11, nylon 12, etc. can be used for the polyamide powder.Nylon 11 and nylon 12, among others, are preferred because of their goodcompatibility with adhesive agent or tackifier.

Example of the polyamide-imide powder includes a condensation product oftrimellitic acid derivative and aromatic diamine. Example of thetrimellitic acid derivative includes trimellitic anhydride; trimelliticanhydride monochloride; 1,4-dicarboxy-3-N,N-dimethyl carbamoyl benzene;1,4-dicarbomethoxy-3-carboxy benzene; 1,4-dicarboxy-3-carbophenoxybenzene; and ammonium salts formed of trimellitic acid, ammonia,dimethylamine, triethylamine, etc. Among these, trimellitic anhydride,trimellitic anhydride monochloride, etc. are often used with preference.Example of the aromatic diamine includes2,2-bis[4-(4-aminophenoxy)phenyl] propane; 2,2bis[4-(4-aminophenoxy)phenyl] butane;1,1-bis[4-(4-aminophenoxy)phenyl]cyclohexane;1,1-bis[4-(4-aminophenoxy)phenyl]cyclopentane;bis[4-(4-aminophenoxy)phenyl]sulfone;bis[4-(4-aminophenoxy)phenyl]ether; 4,4′-carbonyl bis(P-phenylene oxy)dianiline, etc.

Among these, 2,2-bis[4-(4-aminophenoxy)phenyl] propane is preferred.Whenever necessary, a mixture of the above diamine may be used.Furthermore, the already known diamine, for example, 4,4′-diaminodiphenyl ether; 4,4′-diamino diphenyl methane; 4,4′-diamino diphenylsulfone; meta-phenylene diamine; piperazine; hexamethylendiamine; heptamethylendiamine; tetramethylendiamine; para-xylylendiamine;meta-xylylendiamine; 3-methyl heptamethylenediamine;1,3-bis(3-aminopropyl)tetramethyl disiloxiane, etc. may be used inaddition. For the adhesive agent, tackifier and resin in the presentinvention, following items can be used.

The adhesive agent and tackifier may be selected from among the group ofnatural rosin (gum rosin, wood rosin, tall oil rosin, etc.), denaturedrosin

(polymerized rosin, hydrogenated rosin, maleic acid modified rosin,etc.); cumarone-indene resin; terpene system resin; petroleum systemresin; and phenol system resin, etc. The resin may be selected fromamong the group of ethylene vinyl acetate copolymer; ethylene acrylicacid copolymer; ethylene ethyl acrylate copolymer; ethylene methylacrylate copolymer; ethylene propylene copolymer having a molecularweight higher than 30,000 or ethylene butene copolymer; poly butene;wax, etc. These items may be used either in solo, or in a combination oftwo or more.

The tackifier expedites thermal plastic deformation of polyamide powderand improves the wetting property of junction surface during compressionof compound. In this way, the thermal compression bonding property israised with the polyamide powder, or epoxy oligomer. Plasticizer is usedwhenever necessary. It lowers viscosity of the polyimide powdercontaining adhesive agent, and improves the flexibility and the wettingproperty. Practical example includes dibenzyl toluenes, p-hydroxybenzoate, benzenesulfonamides, etc., which are the compounds having arelatively good compatibility with the polyamide powder.

From the view point of the compatibility and the plasticizationefficiency, an addition product of glycidyl compound and carboxylic acidhaving the following structure is more preferred.

where R1 and R2 signify at least one hydrocarbon group selected fromamong the group of aliphatics, alicyclics and aromatics. At least eitherone of R1 and R2 is an aromatic hydrocarbon group.

From the view point of practical usage, the above-described examples ofaddition product are divided into following three categories: (A) anaddition product of aliphatic glycidyl ether, or alicyclic glycidylether, and aromatic carboxylic acid, (B) an addition product of aromaticglycidyl ether and aliphatic carboxylic acid, or alicyclic carboxylicacid, and (C) an addition product of aromatic glycidyl ether andaromatic carboxylic acid. The compounds of each of the above categoriesare provided through the following procedure: an exemplary process is;adding a certain specific carboxylic acid into corresponding glycidyilether in the presence of a catalyst, heating and stirring at atemperature 60-120□ in the normal atmospheric pressure, or in a reducedpressure. As for the catalyst, a tertiary amine, an imidazole, a metalester system compound, a phosphorus system compound, etc. can be used.Polyimide powder mixed with plasticizer is also provided withstickiness.

In order to intensify the bonding force with binder, it is preferred tocover the surface of rare earth system magnetic powder beforehand withan epoxy oligomer, which is solid in the normal room temperature.Average thickness of the surface coating should be less than 0.1 μm.This is important in order to prevent a deterioration with theorientation, which is caused by secondary mutual cohesion withinmagnetically anisotropic rare earth system magnetic powder.

Method of covering rare earth system magnetic powder with epoxy oligomeris: First dissolve the relevant epoxy oligomer into an organic solvent,and wet-mix it with the rare earth system magnetic powder. And then,crush the massive mixture after removing the solvent. In order to havean increased cross-linking density with the epoxy oligomer, a novolaktype epoxy, which has epoxy group in the molecular chain too, ispreferred. As for the powder epoxy hardener for cross-linkage with theepoxy oligomer, at least one item selected from among the group ofdicyanodiamide and the derivative, dihydrazide carboxylate, anddiaminomaleonitrile and the hydrazide derivative is used. These aregenerally high-melting point compounds which do not dissolve easily inan organic solvent. Preferred grain diameter of which is within a rangeof several μm to 100 μm. The dicyanodiamide derivative includes, forexample, ortho-tolylbiguanide, α-2,5-dimethylbiguanide,α-ω-diphenylbiguanide, 5-hydroxybutyl-1-biguanide, phenylbiguanide,α,ω-dimethylbiguanide, etc. As for the dihydrazide carboxylate,hydrazide succinate, hydrazide adipate, hydrazide isophthalate,4-hydroxy benzoic hydrazide, etc. can be used. These hardeners shouldpreferably be added to the compound through a dry-blending process.

In order to avoid sticking of compound onto molding die, it is preferredto use a lubricant. As for the lubricant, at least one item selectedfrom among the group of higher fatty acid, higher fatty acid amide andmetal soap having a melting point higher than the temperature set forthe molding die is used. Amount of the lubricant should be not more than0.2 weight %, and applied to the compound preferably through adry-blending process.

Preferred amount of the rare earth system magnetic powder contained inthe compound is 92-97 weight %, the compression pressure to be not lessthan 4 ton/cm2. The curing temperature of green sheet should be atemperature that is higher than the relevant epoxy oligomer's reactionstarting temperature. In the final rolling step, the rolling rate shouldbe set at 2% or higher, the marginal wrap-around diameter to be 8 mm orless; or rolling rate at 10% or higher, the marginal wrap-arounddiameter to be 2 mm or less. By so doing, a flexible bonded magnet thatprovides a good matching between the mechanical strength and themagnetic characteristics is implemented.

In order to reduce the cogging torque with high efficiency small motors,green sheets are ultimately built within the motors, the green sheet maybe produced to a finished state where the width is uneven or thicknessis not homogeneous. While on the other hand, it is difficult to producea green sheet into an uneven width or uneven thickness, through theprocess of rolling or extrusion.

Now, the magnetic powder used in the present invention is described.Examples of magnetically isotropic rare earth magnetic powder include aNd—Fe—B system spherical powder produced by the spinning cup atomizationprocess; a Nd—Fe—B system flake powder, an αFe/Nd—Fe—B system flakepowder, a Fe3B/Nd—Fe—B system flake powder, a Sm—Fe—N system flakepowder, an αFe/Sm—Fe—N system flake powder, produced by the meltspinning process. Ratio of grain diameter to thickness of these flakepowder should preferably be lower than 4. As for the magneticallyanisotropic rare earth system magnetic powder, a Nd—Fe—B system massivepowder produced by the hot upsetting process, a magnetically anisotropicrare earth Nd—Fe—B system massive powder produced by the HDDR(Hydrogenation-Disproportionation-Desorption-Recombination) process andthe like powders may be used.

The above-described powders whose surface has been processed to be inertusing a photo-decomposition Zn_can also be used. Coercive force at 20°C. of these magnetic powders after a 4 MA/m pulse magnetization shouldpreferably be not lower than 1.1 MA/m. As for the magneticallyanisotropic rare earth system magnetic powder, a Sm—Fe—N system finepowder having magnetically anisotropic property produced by the RD(Reductive Diffusion) process, and the above powder whose surface hasbeen processed to be inert can also be named. Coercive force at 20° C.of these powders after a 4 MA/m pulse magnetization should preferably benot lower than 0.6 MA/m.

The above-described rare earth system magnetic powders may be usedeither in solo or as a mixture of two or more. However, it is preferredthat average value of coercive force at 20° C. of the entire mixed rareearth magnetic powders after a 4 MA/m pulse magnetization is not lowerthan 0.6 MA/m. After it is finally rolled, the surface may be providedwith a hot-melt type self-bonding layer, or a self-bonding layer havinga film-forming function formed of one or two, or more, types of polymermixed with a blocked isocyanate.

Through the above-described measures, ease of mounting the bondedmagnets in permanent magnet type motors can be raised. Using theflexible bonded magnet in the present invention for the stator or therotor, various types of permanent magnet type motors can bemanufactured. For example, a permanent magnet type DC motor whosepermanent field magnet is a flexible bonded magnet curled in a ringshape, fixed along the inner circumference of a cylindrical frame andmultipole-magnetized; a permanent magnet type motor which uses aring-shape flexible bonded magnet disposed along the inner circumferenceof a cylindrical frame as the radial-gap type rotor; and a permanentmagnet type motor which uses a ring-shape flexible bonded magnetdisposed along the outer circumference of a cylindrical frame as thesurface-magnet type rotor.

Among these types of motors, a permanent magnet type motor using aflexible bonded magnet whose MEP in the normal room temperature after a4 MA/m pulse magnetization is higher than 140 kJ/m3 seems to bepromising as the next generation version of the permanent magnet typemotors based on a combination of Nd—Fe—B system flake powder made by themelt spinning method, rigid thermosetting resin and compression process.A permanent magnet type motor using a flexible bonded magnet whose MEPin the normal room temperature after a 4 MA/m pulse magnetization ishigher than 40 kJ/m3 seems to be promising as the next generationversion of the permanent magnet type motors based on ferrite rubbermagnets.

First Exemplary Embodiment

An exemplary embodiment of the present invention is detailed below. Itis to be noted that the scope of the present invention is not limited bydescription of embodiments. The drawings of motors have been prepared toprovide the concept; they are not intended to offer precise actualdimensions.

A process of manufacturing a flexible bonded magnet is described withreference to FIG. 4. In the present embodiment, three kinds of rareearth system magnetic powders are used for the manufacturing. The firstone is magnetically anisotropic Nd—Fe—B system massive powder producedby the HDDR process (composition: Nd12.3 Dy0.3 Fe64.7 Co12.3 B6.0 Ga0.6Zr0.1). Hereinafter this is referred to as magnetic powder A; averagegrain diameter of which is 55 μm. The second one is Nd—Fe—B systemspherical powder produced by the spinning cup atomization process(composition: Nd13.3 Fe62.5 B6.8 Ga0.3 Zr0.1). This one is referred toas magnetic powder B; average grain diameter is 80 μm. The third one isNd—Fe—B system flake-shape powder produced by the melt spinning process(composition: Nd12 Fe77 Co5 B6). This one is referred to as magneticpowder C; average grain diameter is 80 μm.

Next, binder system is described. It is formed of a novolak type solidepoxy oligomer in the normal room temperature, a latent epoxy hardenerin powder state whose grain diameter is smaller than 15 μm, a polyamidepowder frozen-crushed beforehand to be smaller than 100 μm and containsan adhesive agent, and a lubricant whose grain diameter is smaller than10 μm. The latent epoxy hardener in powder state has the followingchemical structure:(NH₂NHCOCH₂CH₂)₂N(CH₂)₁₁CONHNH₂Dissolve the relevant epoxy oligomer in an organic solvent (acetone) toprepare an epoxy resin solution. Using a sigma blade mixer, wet-mix themagnetic powder A with a certain amount of the resin solution so thatthe solid ingredient occupies 0.5 weight %. Heat the wet mixture to60-80° C. to have solvent evaporated. Crush the dried massive mixture.

If the epoxy resin surface coat is thinner than approximately 0.1 μm,there is hardly any change in the grain diameter distribution with themagnetic powder A before and after the surface coating. The same appliesalso to the magnetic powders B and C. There is hardly any change in theafter-curing strength even if the average coating thickness is increasedup to 0.2 μm.

And then, dry-mix, using a mortar in the normal room temperature, themagnetic powder A having the epoxy oligomer-surface coat with polyamidepowder containing 20% adhesive agent for 3-7 weight %, hardener for 0.05weight %, and lubricant (calcium stearate, grain diameter less than 10μm) for 0.05 weight %. In this way, compound in the powder state isprovided. Because of the lubricant contained, this compound has acertain powder fluidity that is suitable for processing in a powderprocessing facility. The powder-state compound maintains a superiorstorage stability for a long time in the normal room temperature. Anyone of the adhesive agents may be used.

The terminology normal room temperature in the present invention means atemperature range 15-40° C. Next, apply the powder compound in a diecavity of powder compacting press. The molding die is locally heated to70-80° C., only at the upper and the lower punches and the neighborhoodof cavity. The powder compound disposed in the cavity is compressed bythe upper and lower punches under the influence of a 1.4 MA/m axialmagnetic field. After demagnetization, a flexible green sheet is takenout of the machine in a near-net shape. Although the green sheet issoft, there is no practical problem in handling it. The green sheet isheated for 20 min. at 180° C. for curing. A bonded magnet is thusprovided. It is finally rolled with revolving roller heated at 60-80°C., to produce a sheet-formed flexible bonded magnet of 2-20% rollingrate. Likewise, the magnetic powder B and magnetic powder C can beprocessed into sheet-formed flexible bonded magnets. However, nomagnetic field is applied during compression process on the magneticallyisotropic magnetic powders B and C.

For comparison, following two magnet samples are produced representingthe widely-used magnets in small motors: a magnet produced based on thecombination of a ferrite system magnetic powder/a flexible resin/arolling or extrusion process (this type of magnet is hereinafterreferred to as conventional ferrite bonded magnet); and a magnetproduced based on the combination of the magnetic powder C/a rigidthermosetting resin such as epoxy resin/a compression process(hereinafter referred to as conventional rare earth system bondedmagnet). Each of the magnets thus prepared are evaluated on with respectto the mechanical properties such as elongation, tensile strength, etc.,the magnetic characteristics and irreversible flux loss derived fromdemagnetization curve, and other items. Furthermore, ease of mountingthe flexible bonded magnet in small DC motors is evaluated throughappraisal of the windability around a mandrel. The torque constant,which being a fundamental characteristic of motor, is also evaluated on.

FIG. 2 shows a relationship between the molding pressure and the densityof green sheet. Where, curve A93 represents a green sheet containing themagnetic powder A for 93 weight %. Likewise, curve B93 and curve B97represent, respectively, those green sheets containing the magneticpowder B for 93 weight % and 97 weight %. Curve C93 represents a greensheet containing the magnetic powder C for 93 weight %. Curve A97 isoverlapping with the curves A93 and B93, so it is difficult todistinguish from each other. The green sheet measures 6.1 mm wide, 65 mmlong, 1.1 mm thick, and the molding die temperature is 60-80° C.

As seen in the chart, the density almost saturates at a pressureapproximately 400 MPa. Conventional rare earth bonded magnets requireapproximately 980 MPa for obtaining an identical density (approximately5.8 Mg/m3). A flexible bonded magnet in the present invention realizes acomparable density with a lower pressure. The reason is that thepolyamide grains have been softened by a 60-80° C. heat, and asufficient plastic deformation has been taken place during the compoundcompression. Thanks to the low pressure, the rare earth system magneticpowder is damaged less during the compression process. As a result,deterioration of magnetic characteristics due to emergence of activesurface can be reduced.

FIG. 3 shows a relationship between the quantity of the magnetic powdersA, B and C and the density of flexible bonded magnet in the presentinvention. Where, curve A represents a bonded magnet produced from themagnetic powder A, curve B a bonded magnet produced from the magneticpowder B, and curve C a bonded magnet produced from the magnetic powderC. Each magnet measures 6.1 mm wide, 65 mm long and 1.1 mm thick.Compressing pressure at 60-80° C. is 490 MPa, curing conditions are 180°C. for 20 min.

As seen in FIG. 2 and FIG. 3, the magnetic powders A and B, whose grainshapes are close to a sphere, exhibit a higher density, hence it iseasier to fill-in than the flake-shape magnetic powder C. Despite theabove difference, the present embodiment is applicable to any types ofthe rare earth system magnetic powders, and implements a flexible bondedmagnet whose density is higher than any of the known rubber magnets.

FIG. 4A shows a relationship between the tensile strength (relativevalue) and the heating temperature applied on green sheets made from themagnetic powders A, B and C. FIG. 4B shows a relationship between thetensile strength and the heating time of green sheets made from themagnetic powders A, B and C. In the charts, curve A represents a greensheet made from the magnetic powder A, curve B a green sheet made fromthe magnetic powder B and curve C a green sheet made from the magneticpowder C. Each magnet measures 6.1 mm wide, 65 mm long, 1.1 mm thick.Compression pressure at 60-80° C. is 490 MPa. No hardener is used inthose shown in FIG. 4A and FIG. 4B.

The optimum curing temperature in the present embodiment is, as shown inFIG. 4A, approximately 180° C., regardless of whether it is of themagnetic powder A, B or C. Tensile strength of a flexible bonded magnetin the present invention increases to 2.5-4 times that at the greensheet state. The increase of tensile strength is observed after heatingat a temperature higher than 120° C. This seems to have been caused by acuring reaction taken place between the epoxy group of epoxy oligomercontained in the compound in powder state and the active hydrogen inpolyamide powder. Considering the heating time shown in FIG. 4B, theoptimum curing conditions in the present embodiment seem to be 180° C.for 10-30 min.

Next, control of bonded magnet's flexibility by means of rolling isdescribed. General practice of assembling a flexible bonded magnet in asmall motor is first winding it around. So, the flexibility control isan important item for the flexible bonded magnets. The rolling processprovides an effective tool for controlling the flexibility. The upperand lower rolls used have a 90 mm diameter, the roller temperature is70° C. Flexibility is evaluated in terms of limit_diameter of a mandrel,around which a magnet can wind. The smaller the limit diameter, thegreater is the flexibility. The dimensional change caused in a flexiblebonded magnet as a result of rolling is also evaluated on.

FIG. 5 shows a relationship among the rate of reduction due to rolling,the windability of a flexible bonded magnet, and the dimensional changecaused in the flexible bonded magnet. The lateral axis representsrolling rate, while the left longitudinal axis representing windablelimit diameter and width, the right longitudinal axis representingmagnet length. In the chart, curve D represents the windability, curve Ethe magnet length, and curve F the magnet width. The sample flexiblebonded magnet is the one produced from the magnetic powder B (contents:95 weight %). Rolling temperature is 70° C. Rolling rate is calculatedon the basis of ratio of magnet thickness before and after the rolling.The magnet measures 6.1 mm wide, 65 mm long, and 1.1 mm thick.

As seen in the chart, the windability rapidly improves with those rolledat a rolling rate 2-10%. Moreover, the dimensional change of a magnetremains very small when the rolling rate is within a 2-10% range. Forexample, a 1.2 mm thick magnet can be bent to wind around a mandrel evensmaller than 8 mm in the diameter. On the other hand, when the rollingrate is higher than 10%, the magnet dimension significantly increases inthe rolling direction (length). Thus, a ring magnet smaller than 4 mmdiameter can be manufactured by bending a 1.1 mm thick magnet using amandrel of 2 mm diameter.

FIG. 6 shows a relationship between the mechanical property of aflexible bonded magnet in the present invention and the rolling rate.Where, curve G represents the elongation, while curve H the tensilestrength. The sample magnet is the one produced from the magnetic powderB (content: 95 weight %). Rolling temperature is 70° C., rolling rate iscalculated on the ratio of magnet thickness before and after therolling. The magnet measures 6.1 mm wide, 65 mm long, and 1.1 mm thick.As seen in the chart, the elongation of magnet in the rolling directionincreases in an approximate proportion to the rolling rate. The tensilestrength of magnet decreases inverse-proportionate to the rolling rate.Taking the elongation and the tensile strength into consideration, therolling rate has been determined to be 5-10%. The ring magnets thusmanufactured can be used in the actual motors.

FIG. 7 shows a relationship between the amount of rare earth systemmagnet powder contained in a flexible bonded magnet in the presentinvention and the elongation before and after the rolling. Where, curveJ represents the elongation after it is rolled at a 10% rolling rate,curve I representing the elongation before rolling. The flexible bondedmagnet sample is the one produced from the magnetic powder A (content:93-95 weight %). Rolling temperature is 70° C. Rolling rate iscalculated based on the ratio of magnet thickness before and after therolling. The magnet measures 6.1 mm wide, 65 mm long, and 1.1 mm thick.

As seen in the chart, the elongation of magnet decreases when content ofthe rare earth system magnetic powder exceeds 97 weight %. However, therolling process brings about a flexibility. As the result, a 1.1 mmthick magnet can be wrapped around a mandrel of 10 mm diameter. As isalready known, filling amount of the 97 weight % rare earth systemmagnetic powder is identical to that of conventional rare earth bondedmagnet. Namely, the present invention provides an advantage that thesame density level is made available by a lower compression pressure.Furthermore, elongation and tensile strength in the normal roomtemperature of a flexible bonded magnet in the present invention were inapproximately the same level as those of conventional ferrite bondedmagnets used in the general-use small motors.

FIG. 8 and FIG. 9 show demagnetization curve and B-H curve,respectively, of four kinds of magnets; viz. flexible bonded magnets inthe present invention produced from the magnetic powder A and themagnetic powder B, a conventional ferrite bonded magnet and aconventional rare earth element bonded magnet. The magnets measure 7.5mm wide, 7.5 mm long and 1.1 mm thick. Curve A and curve B represent,respectively, the flexible bonded magnets produced from the magneticpowder A and the magnetic powder B in the present invention; curve C aconventional rare earth element bonded magnet; and curve D aconventional ferrite bonded magnet. All the magnets were measured afterthey were magnetized in a 4 MA/m pulse magnetic field. The comparativemagnet samples are those generally used for small DC motors or smallbrushless motors.

As seen in the charts, the flexible bonded magnets A and B in thepresent invention provide an evidently higher magnetic flux density atthe gap between rotor iron core and stator magnet, as compared withthose of conventional magnets C and D.

Describing practically, as shown in FIG. 14, MEP of a flexible bondedmagnet produced from the magnetic powder A in accordance with thepresent invention (density: 5.84 Mg/m3) is 140 kJ/m3; namely, it is 1.75times that the conventional rare earth element bonded magnet, 80 kJ/m3.MEP of a flexible bonded magnet produced from the magnetic powder B inaccordance with the present invention (density: 5.45 Mg/m3) is 40 kJ/m3;namely, it is more than 3 times as high that the conventional ferriterubber magnet. When the magnet is used as a ring magnet smaller than 10mm diameter, it improves the efficiency of small motors.

FIG. 10 shows demagnetization curves of flexible bonded magnets producedfrom the magnetic powder C in the present invention. Where, curve C1represents a magnet of 5.4 Mg/m3 density, C2 a magnet of 5.96 Mg/m3density. These magnets have almost the same coercive force, 690 kA/m,because both are made from the same magnetic powder C. However,reflecting the difference in density, MEP of one magnet is 66 kJ/m3,while another magnet shows approximately 80 kJ/m3. The MEP value, 80kJ/m3, is identical to that of conventional rare earth element bondedmagnets. Although the bonded magnet in the present invention is aflexible bonded magnet, its density can be raised to a level as high asshown in FIG. 2 and FIG. 3; so, it exhibits superior magneticcharacteristics.

Further advantage is that the magnets in the present invention can bemanufactured on the conventional existing processing machines.Furthermore, since the flexible bonded magnets in the present inventioncan be bent or rolled thinner easily, they can be used widely in thehigh-efficiency small motors. Still further, since the magnets havesolved a crack problem which the conventional rigid magnets can nottotally get rid of, they provide a high reliability.

FIG. 11 shows the temperature dependencies of the initial irreversibleflux-loss of flexible bonded magnets in the present invention. Curve Arepresents a flexible bonded magnet produced from the magnetic powder A,curve B that produced from the magnetic powder B, and curve C thatproduced from the magnetic powder C. Temperature of motor magnets inmost of the operating high-technology apparatus is lower than 100° C.

So, as seen in the chart, initial irreversible flux-loss of the flexiblebonded magnets in the present invention is small, in so far as they areoperating in an environment where the magnet temperature can bemaintained to be lower than 110° C. However, in order to ensure such afavorable magnetic stability, it is preferred that the coercive force inthe normal room temperature of rare earth system magnetic powder after a4 MA/m pulse magnetization is 600 kA/m or higher.

In the case of a magnetically anisotropic flexible bonded magnetproduced from the magnetic powder A, the temperature coefficient ofcoercive force is approximately −0.5%/° C.; namely, it is greater thanthat of generally-used rare earth system magnetic powder, −0.4%/° C.Because of this, it is preferred that in the case of magneticallyanisotropic magnetic powder A the coercive force in the normal roomtemperature after a 4 MA/m pulse magnetization is 1.1 MA/m or higher.Meanwhile, a flexible bonded magnet in the present invention which hasbeen produced from an anisotropic rare earth system magnetic powder,such as the magnetic powder A, might have a problem of long termmagnetic flux loss, the problem which an anisotropic rigid epoxy bondedmagnet has.

Conventionally, the rigid epoxy bonded magnets were produced bycompressing a magnetically anisotropic rare earth system magneticpowder, such as the magnetic powder A, with a compression pressure ashigh as approximately 980 MPa. On the other hand, a flexible bondedmagnet in the present invention is manufactured with a low compressionpressure which as low as approximately 40% of the conventionally-usedpressure. As the result, damage on the magnetic powder A and theresultant emergence of new surfaces decrease. Therefore, the long termmagnetic flux loss with the flexible bonded magnet produced from themagnetic powder A seems to decrease further in an operating temperaturerange lower than 100° C. As described earlier, the bonded magnet in thepresent invention has a favorable wrap-around property for forming aring-shape magnet, and superior magnetic characteristics.

FIG. 12 shows a cross sectional view of a small DC motor incorporating aflexible bonded magnet produced in accordance with the presentinvention. The motor includes shaft 30, bearing 32, frame 34, rotor 42,stator (magnet) 50, slot 40, and teeth 41. Flexible bonded magnet 50 hasbeen shaped to a ring form, wrapped in iron frame 34, and magnetized toprovide a permanent field magnet. Rotor 42 is inserted to complete afinished motor. The motor measures 24 mm in diameter, 12.5 mm in height.

The table below shows the torque constant of four types of small DCmotors, incorporating, respectively, the flexible bonded magnetsproduced from the magnetic powder A and the magnetic powder B in thepresent invention, a conventional rare earth element bonded magnet and aconventional ferrite bonded magnet. The torque constant being thefundamental item of an output; the torque constant is shown standardizedin the table with that of a DC motor incorporating ferrite bonded fieldmagnet as the reference. Torque constant of a motor using a flexiblebonded magnet produced from the magnetic powder A for the field magnetis 1.49 times as high compared with a motor incorporating conventionalrare earth element bonded magnet. It is 3.12 times as high compared witha motor incorporating conventional ferrite bonded field magnet. Torqueconstant of a motor using a magnet produced from the magnetic powder Bfor the field magnet is 1.79 times as high compared with a motorincorporating a conventional ferrite bonded field magnet.

Magnetic Magnetic Conventional Conventional Powder A Powder B RE MagnetFerrite Magnet Ratio of 3.12 1.78 2.09 1 Torque Constant RE: Rare Earth

In some of the DC motors, cogging torque sometimes may increase whenmagnetic flux density is high at the air gap between rotor core iron andstator magnet. The cogging torque here means a pulsating torque causedby a change in the permeance coefficient due to rotor revolution;because there are teeth 41 and slots 40 on the outer circumferentialsurface of rotor core iron facing the stator. This sometimes canincrease vibration and noise of a motor, or ill-affect the accuracy inposition control. In a flexible bonded magnet in accordance with thepresent invention, however, the green sheet can be processed to have anuneven width or uneven thickness easily; this can be utilized as meansto bring the air gap between rotor core iron and stator to exhibit awaveform that is closer to the sine waveform, thereby suppressing apossible increase of cogging torque. The conventional method ofprocessing a flexible sheet magnet, such as rolling, extrusion, etc.,can not reasonably meet the above-described way for the solution. In thepresent invention, however, a compound in powder state can be compressedinto a green sheet form, the approximate thickness of which is normally0.5-2.0 mm; so, there is a wide flexibility for the green sheet to adaptitself to any desired specific shapes and dimensions for meeting adesign concept of a motor.

Second Exemplary Embodiment

In the present embodiment, a green sheet containing soft magnetic powderand a green sheet containing rare earth system magnetic powder areintegrated into a single sheet. The sheet is cured, and rolled toprovide a complex flexible bonded magnet of soft magnetic material.Since the complex flexible bonded magnet of soft magnetic material hasbeen formed by unitizing a soft magnetic layer and a magnet withoutusing an intervening adhesive layer, it can constitute a high-efficiencymagnetic circuit.

Describing the process practically; prepare, a compound which containsat least one kind of soft magnetic powder selected from among the groupof those having saturation magnetization 1.3 T or higher, Fe, Fe—Ni,Fe—Co, Fe—Si, Fe—N and Fe—B, in place of the rare earth system magneticpowder in the first embodiment. Produce a green sheet from the compound,and set the green sheet in a cavity of molding die. And then, fill thecavity with the first embodiment's compound in powder state, andcompress them together. In this way, a complex body of green sheetshaving different functions is provided. Cure and roll the complex greensheet, then a flexible bonded magnet with back yoke is provided.

Another exemplary application is; compressing part of the binderingredients in advance, and then filling the cavity with the firstembodiment's compound in powder state for compression. A flexible bondedmagnet having thermal bondability is thus produced. Still another,forming in advance one or more kinds of self-adhesive layer on thesurface of the flexible bonded magnet by mixing blocked isocyanate witha polymer having film-forming function, such as bisphenol A type epoxyresin. By so doing, it can be used for joining the magnet ends togetheror with other material.

The flexible bonded magnet in the present invention can solve a problemof orientation deterioration with a magnet which has been manufacturedin the radial orientation of magnetic field caused as the result ofbending around a small diameter circle, namely a problem ofdeterioration in the magnetic characteristics. As described in theabove, the present invention can offer a high-performance ring magnetwhose MEP is as high as 140 kJ/m3 in radius direction, irrespective ofthe bending diameter. As viewed from the manufacturing facility,productivity of the magnet is high.

Furthermore, the present invention is applicable to any kinds of rareearth system magnetic powders. For example, using a magneticallyisotropic rare earth system magnetic powder magnets of MEP 40 kJ/m3level can be produced economically. It contributes also to improveefficiency of the low-efficiency small motors built with ferrite rubbermagnets. The present invention does not employ a process temperaturehigher than 200° C., that was needed in the conventional manufacturingprocesses (rolling, extruding). This, too, contributes to a highmanufacturing productivity. Thus the present invention will make acontribution to downsize the high-technology devices, and save theelectric powers and natural resources.

INDUSTRIAL APPLICABILITY

A method of manufacturing a flexible bonded magnet in accordance withthe present invention is not restricted by the magnetic powders used.Motors incorporating the magnets exhibit a high efficiency that ishigher than that of motors using conventional bonded magnets producedfrom ferrites, rare earth element magnetic powders, etc.

1. A method of manufacturing a flexible bonded magnet comprising thesteps of compressing a compound consisting of a) rare earth systemmagnetic powder and b) flexible thermosetting resin composite,heat-curing a green sheet derived from the above step, and rolling thegreen sheet.
 2. The method of manufacturing a flexible bonded magnetrecited in claim 1, wherein the flexible thermosetting resin compositeincludes an solid epoxy oligomer in the normal room temperature and apolyamide powder having thermo-compression bonding property providedwith stickiness in the normal room temperature.
 3. The method ofmanufacturing a flexible bonded magnet recited in claim 2, wherein theflexible thermosetting resin composite contains at least one kind ofadhesive agent.
 4. The method of manufacturing a flexible bonded magnetrecited in claim 2, wherein at least either one among a powder-statelatent epoxy hardener and a lubricant is further used.
 5. The method ofmanufacturing a flexible bonded magnet recited in claim 2, furthercomprising a process for covering beforehand the surface of the rareearth system magnetic powder with the epoxy oligomer.
 6. The method ofmanufacturing a flexible bonded magnet recited in claim 2, furthercomprising a process of dissolving the epoxy oligomer in a solvent andwet-mixing with the rare earth system magnetic powder, and a process ofcrushing after removing the solvent.
 7. The method of manufacturing aflexible bonded magnet recited in claim 2, wherein the epoxy oligomer isa novolak type epoxy resin.
 8. The method of manufacturing a flexiblebonded magnet recited in claim 2, wherein the flexible thermosettingresin composite contains an addition product of glycidyl compound andcarboxylic acid.
 9. The method of manufacturing a flexible bonded magnetrecited in claim 4, wherein the powder-state latent epoxy hardener is adihydrazide system compound.
 10. The method of manufacturing a flexiblebonded magnet recited in claim 4, wherein the lubricant is at least oneitem selected from the group of a higher fatty acid, a higher fatty acidamide and a metal soap whose melting points are higher than the moldingdie temperature.
 11. The method of manufacturing a flexible bondedmagnet recited in claim 1, wherein content of the rare earth systemmagnetic powder falls within a range 92 weight %-97 weight %.
 12. Themethod of manufacturing a flexible bonded magnet recited in claim 1,wherein rolling rate at the rolling process is not lower than 2%, andwindable limit diameter is not larger than 8 mm.
 13. The method ofmanufacturing a flexible bonded magnet recited in claim 1, whereinrolling rate at the rolling process is not lower than 10%, and windablelimit diameter is not larger than 2 mm.
 14. The method of manufacturinga flexible bonded magnet recited in claim 1, wherein the green sheet isshaped to have at least either one item, an uneven width or an uneventhickness.
 15. The method of manufacturing a flexible bonded magnetrecited in claim 1, wherein the rare earth system magnetic powder is amagnetically isotropic Nd—Fe—B system spherical powder produced by thespinning cup atomization method.
 16. The method of manufacturing aflexible bonded magnet recited in claim 1, wherein the rare earth systemmagnetic powder is a magnetically isotropic Nd—Fe—B system flake-shapepowder produced by the melt spinning method.
 17. The method ofmanufacturing a flexible bonded magnet recited in claim 1, wherein therare earth system magnetic powder is at least one magnetically isotropicflake-shape powder selected from among the group of α-Fe/Nd—Fe—B system,Fe3B/Nd—Fe—B system, Sm—Fe—N system, and α-Fe/Sm—Fe—N system, producedby the melt spinning method.
 18. The method of manufacturing a flexiblebonded magnet recited in claim 1, wherein the rare earth system magneticpowder is a magnetically anisotropic Nd—Fe—B system massive powderproduced by at least either one of the hot upsetting method and the HDDRmethod.
 19. The method of manufacturing a flexible bonded magnet recitedin claim 18, wherein coercive force at 20° C. of the rare earth systemmagnetic powder after a 4 MA/m pulse magnetization is not lower than 1.1MA/m.
 20. The method of manufacturing a flexible bonded magnet recitedin claim 1, wherein the rare earth system magnetic powder is amagnetically anisotropic Sm—Fe—N system fine powder produced by the RD(Reductive Diffusion) method.
 21. The method of manufacturing a flexiblebonded magnet recited in claim 20, wherein coercive force at 20° C. ofthe magnetically anisotropic Sm—Fe—N system fine powder after a 4 MA/mpulse magnetization is not lower than 0.6 MA/m.
 22. The method ofmanufacturing a flexible bonded magnet recited in claim 1, furthercomprising a process for forming a self-bonding layer on the surface,following the rolling process.
 23. The method of manufacturing aflexible bonded magnet recited in claim 22, wherein the self-bondinglayer is a hot melt type.
 24. The method of manufacturing a flexiblebonded magnet recited in claim 22, wherein the self-bonding layercontains at least one kind of polymer which has a film-forming functionand mixed with a blocked isocyanate.
 25. A method of manufacturing apermanent magnet type motor, comprising the steps of: compressing acompound consisting of a) rare earth system magnetic powder and flexiblethermosetting resin composite; heat-curing a green sheet derived fromthe above step; rolling the green sheet; forming a self-bonding layer ona surface to form a flexible bonded magnet; and joining the flexiblebonded magnet with a counterpart material including said magnet in saidmotor.
 26. A method of manufacturing a permanent magnet type motor,comprising the steps of: compressing a compound consisting of a) rareearth system magnetic powder and b) flexible thermosetting resincomposite; heat-curing a green sheet derived from the above step;rolling the green sheet; forming a self-bonding layer on a surface toform a flexible bonded magnet; and joining both ends of the flexiblebonded magnet wound-around to a ring shape.